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Table 6.-Number of phytoplankton species found in the <25-µm and >25-µm size-fractions on the lower Colorado River. Station Bacillariophyta Chlorophyta Chrysophyta diatoms green algae golden-brown algae Cyanophyta Euglenophyta Pyrrhophyta Rhodophyta blue-green euglenoids dinoflagellates red algae algae Total species <25-µm Size-fraction Davis Dam 45 9 2 4 1 2 - 63 Park Moabi 59 8 2 4 3 2 - 78 Havasu Delta 73 10 2 8 5 2 - 100 Parker Dam 17 15 3 6 3 4 - 52 Palo Verde Diversion Dam 63 10 3 8 1 3 - 88 Cibola 61 8 2 8 3 - - 82 Palo Verde Irrigation Drain 61 8 - 3 4 - - 76 DR3 65 7 1 6 - 1 - 80 Imperial Dam 81 8 1 4 1 - - 95 All American Canal 67 8 1 4 - - - 79 Laguna 53 8 2 8 2 1 - 74 Yuma 58 16 1 11 3 - - 89 >25-µm Size-fraction Davis Dam 41 9 1 5 - 2 - 58 Park Moabi 42 5 1 3 - 2 - 53 Havasu Delta 60 9 1 7 - 2 1 80 Parker Dam 60 20 3 10 3 2 - 100 Palo Verde Diversion Dam 52 14 3 11 2 2 - 84 Cibola 70 14 3 11 2 3 - 103 Palo Verde Irrigation Drain 45 12 1 10 6 1 - 75 DR3 56 14 2 8 1 4 - 85 Imperial Dam 66 10 1 6 - 3 - 86 All American Canal 62 21 2 10 1 2 - 98 Laguna 69 15 1 10 3 2 - 100 Yuma 59 24 2 10 2 3 - 100 Parker Dam. The greatest number of phytoplankton species (189 species) were collected from the Yuma station. The major taxa of phytoplankton are listed in tables 18A through 25A. Generally, all major taxa decreased in concentration at the downstream stations. The >505-µm size-fraction phytoplankton were dominated by filamentous green algae (i.e., Clado- phora) (tables 26A through 33A). Macrophytes also were present at some stations. Sometimes, Pota- mogeton crispus (curlyleaf pondweed), Potamogeton pectinatus (sago pondweed), and Elodea sp. were very abundant in the samples. Often these macro- phytes were visible in the river channel and formed submergent weed beds. Macrophytes were common in the river from below Davis Dam to Imperial Dam, but less dense at downstream stations. 1986. Calanoids and cyclopoids were collected often at Davis Dam and Parker Dam; generally, densities were below 15 individuals per liter for each sampling date (tables 34A through 41 A). The calanoid species Eurytemora affinis was collected below Davis Dam and Parker Dam and were found in Lake Havasu in 1984 (Roline and Lieberman 1985 [181). The distributional pattern of this calanoid species has changed since it has been reported primarily in Atlantic and Pacific coastal areas, in salt and brackish waters, in the Gulf of Mexico, and in the Great Lakes (Balcer et al. 1984 [19]). At Palo Verde Diversion Dam, adult copepods were collected only once on January 5, 1987. Harpacticoids-known to inhabit both littoral and benthic regions-were often collected upstream but rarely downstream. Cladoc- era were dominated by Daphnia and were present at most stations. Zooplankton.-There were 21 and 16 zooplankton species collected from Davis Dam and Parker Dam, respectively (table 7). Many of these species originated from the reservoirs-Lake Mohave, and Lake Havasu. Nauplii and copepodites were collected below Davis Dam and Parker Dam from October through June 1987; densities were generally below 15 individuals per liter for each sampling date (tables 34A and 36A). These immature forms seldom were collected at downstream stations. Adult calanoids became abundant at Davis Dam only in October During the period, January through June 1987, zooplankton were collected with a 505-µm mesh- size plankton net (tables 26A through 33A). Zooplankton collected in this size-fraction were adult and immature Daphnia forms. Daphnia were abundant (>25 individuals per liter) at Davis Dam, Park Moabi, Parker Dam, and Headgate Rock Dam. Daphnia density decreased from Headgate Rock Dam downstream to Yuma. Juvenile cladoceran densities were greatest below Davis Dam and Parker Dam because of reservoir flows into tailwaters. Rotifer 10